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  • Title: Waxy and non-waxy barley cultivars exhibit differences in the targeting and catalytic activity of GBSS1a.
    Author: Hebelstrup KH, Nielsen MM, Carciofi M, Andrzejczak O, Shaik SS, Blennow A, Palcic MM.
    Journal: J Exp Bot; 2017 Feb 01; 68(5):931-941. PubMed ID: 28199682.
    Abstract:
    Amylose synthesis is strictly associated with activity of granule-bound starch synthase (GBSS) enzymes. Among several crops there are cultivars containing starch types with either little or no amylose known as near-waxy or waxy. This (near) amylose-free phenotype is associated with a single locus (waxy) which has been mapped to GBSS-type genes in different crops. Most waxy varieties are a result of either low or no expression of a GBSS gene. However, there are some waxy cultivars where the GBSS enzymes are expressed normally. For these types, single nucleotide polymorphisms have been hypothesized to represent amino-acid substitutions leading to loss of catalytic activity. We here confirm that the HvGBSSIa enzyme from one such waxy barley variety, CDC_Alamo, has a 90% reduction in catalytic activity. We also engineered plants with expression of transgenic C-terminal green fluorescent protein-tagged HvGBSSIa of both the non-waxy type and of the CDC_Alamo type to monitor their subcellular localization patterns in grain endosperm. HvGBSSIa from non-waxy cultivars was found to localize in discrete concentric spheres strictly within starch granules. In contrast, HvGBSSIa from waxy CDC_Alamo showed deficient starch targeting mostly into unknown subcellular bodies of 0.5-3 µm in size, indicating that the waxy phenotype of CDC_Alamo is associated with deficient targeting of HvGBSSIa into starch granules.
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